During manufacturing, liquid crystal displays (LCDs) have a generally accepted imperfection or defect rate. A significant amount of LCD panels fail initial quality testing based on the amount or type of defects in the LCD panels, resulting in those products being destroyed. However, LCDs with an acceptable amount of defects are kept and ultimately put on sale or otherwise distributed.
Among the defects which are considered acceptable for LCDs are light spot defects and black spot defects. In particular, in dual panel LCDs, such defects are well known and routinely found during manufacturing inspections. In such situations, the LCDs are formed with a first LCD panel stacked on top of a second LCD panel, where the first panel produces a first display from a first image data and the second panel produces a second display from a second display signal that is derived from the first image data. Defective pixels on the first or second LCD panel will then deteriorate the overall quality of the image displayed to a user.
In situations involving a normally white LCD, where pixels are white when voltage is not applied, pixels that are defective allow light to pass through when voltage is applied, causes resulting light spot defects. In other words, instead of the pixels on the white display being driven black when voltage is applied, as intended, the pixels allow the light to pass through them and causing a defect on the displayed image because of the passed-through light. Conversely, in situations involving a normally black LCD, where pixels are black when voltage is not applied, pixels that are defective remain black when voltage is applied instead of being driven white. This results in a black spot defect.
In practical use, light spot defects are more perceptible to the human eye when viewing such a display. Black spot defects, however, are much less perceptible during typical viewing of an LCD. Therefore, there is a higher tolerance for black spot defects on LCDs than there is for white spot defects. As a result, manufactured LCDs with light spot defects above a threshold level are typically scrapped. However, manufactured LCDs with light spot defects below a threshold level are often converted to black spot defects during inspection associated with the manufacture of LCDs. Manufactured LCDs with black spot defects have a higher threshold for the amount of defects before they are scrapped This is considered acceptable due to the black spot defects being less perceptible to a viewer.
However, black spot defects still deteriorate the overall quality of an image displayed on an LCD. In particular, where LCDs are formed with first and second LCD panels, and a back panel has large pixels, the effect of a black spot defect on the front panel is often enhanced. As is often the situation, a back panel of an LCD may have pixels that correspond to three sub-pixels on a front panel, so black spot defects on the back panel can cause significant image deterioration. Thus, and LCD that provides a way of mitigating black spot defects is desired.